The TCAS (Traffic alert and Collision Avoidance System) is installed equipment used for warning a crew of a risk of in-flight collision with another aircraft and for giving it, in time, the information necessary for carrying out an avoidance maneuver. The necessity of such equipment became apparent in the 1950's, but it has taken several decades to achieve maturity in civil aviation. The TCAS in its current form dates from the 1980's and meets the ACAS standards established by the OACI (Organisation de l'Aviation Civile Internationale) and the regulation of the FAA (a government organization responsible for the regulation and control of civil aviation in the United States of America). It makes use of the installed SSR (Secondary Surveillance Radar) transponders responding to the interrogations of the secondary surveillance radars placed on the ground for cooperative air traffic control. These SSR transponders, whose use was made obligatory for all new aircraft of a certain weight or of a certain capacity (15 tonnes or 30 seats for the FAA, 5.7 tonnes or 19 seats for the OACI), correspond to a civil use of IFF (Identification Friend or Foe) systems which appeared during the Second World War.
The TCAS consists of an SSR interrogator associated with a computing unit. The installed SSR interrogator interrogates the SSR transponders installed on board other aircraft flying in the vicinity, whilst the computing unit situates the relative positions of the aircraft returning responses, in bearing using the directional properties of the system of receiving antennas of the SSR interrogator and in distance from the measured delays of the responses.
There are several versions of TCAS. The first version, TCAS 1, meets the ACAS 1 standard of the OACI. It is equipped with an SSR interrogator operating in mode C, that is to say adapted to SSR transponders responding only with an identifier and altitude data. It only provides traffic advisories TA (indications of heading and distance on a screen). The second version, TCAS II, meets the ACAS II standard of the OACI. It is equipped with an SSR interrogator operating in mode S, that is to say adapted to a more developed SSR transponder giving a more selective response provided with more data than mode C since it contains a registration number and supports data transmission. The second version TCAS II remains compatible with mode C. It provides traffic advisories TA, notices of resolution of collision risk RA solely in the vertical plane by means of anti-collision logic tracking targets in relative distance, relative bearing and altitude returned in the C and S mode responses, and coordination interrogations throughout the duration of a resolution advisory RA assuring the compatibility between the avoidance maneuvers of the two aircraft in question. This version is the one currently in use. A third version, the TCAS III, meeting the ACAS III standard of the OACI is foreseen for the future. It is a development of the TCAS II, providing notices of resolution of collision risk in the horizontal and vertical planes.
As TCAS equipments are general on aircraft as soon as they exceed a small transport capacity, it is envisaged to use them for reducing the separation distances on transoceanic routes or during instrument takeoffs as well as for establishing and maintaining the gaps between aircraft at the time of their final approaches.
Given the generalized presence of a satellite positioning receiver on board aircraft, it is also envisaged to use the capability of mode S to transmit data in order to make the on-board TCAS equipment communicate the GPS position of the aircraft to any SSR interrogator and in particular to an air traffic control center, which is advantageous for transoceanic flights.
Returning to transoceanic flights, an aircraft's knowledge of its current geographic location can be rather inaccurate because of an unavailability of its satellite positioning system, of sensor failures, of drifting of the inertial system or of the inertial system/GPS hybridization algorithms. Thus, an aircraft following a transoceanic route can see its estimated position deviate significantly from its real position. As it is rarely alone on a transoceanic route, its safety then depends entirely on its TCAS equipment which monitors its relative position with respect to aircraft flying in its vicinity. The integrity of the TCAS and the reliability of its measurements therefore become preponderant parameters.